https://peakmonsters.com/ has a rental market in place. As far as I know, Splinterlands will soon(ish) be adding one to the official site or adding a gateway to the exisiting rental market on PeakMonsters (I'm not 100% on the details).
I agree with your thoughts on Reward Cards.
!ENGAGE 10
Thanks! I've rented cards before using PeakMonsters and thought the rentals were extremely cheap and worthwhile... but I haven't looked at the prices in the last few weeks. I think I'm going to start renting out my extras...